4.2 Spore germination and dormancy

Spores are products of both sexual and asexual reproduction and act as
the prime units of dispersal in fungi. Most spores that
settle on an appropriate substrate under favourable environmental conditions
germinate to produce one or more germ tubes and a new fungal mycelium.
Except for sterile species, the spores of all fungi can enter into a dormant
phase, during which metabolism is reduced by about 50%. Several types of
dormancy exist in fungi; classed as either exogenous or
endogenous (Feofilova et al., 2012). If a spore is
faced with unfavourable conditions, such as lack of nutrients, low
temperature, an unfavourable pH or the presence of an inhibitor (for
example, on a surface of a plant), the spore remains dormant and delays
germination. Spores under these conditions are exogenously dormant
and will only germinate when environmental conditions become favourable.

Some fungi produce spores that fail to germinate immediately, even under
favourable conditions because of factors within the spore such as nutrient
impermeability or the presence of endogenous inhibitors. Spores of this sort
are said to be endogenously dormant. Dormancy of these
spores is usually broken by ageing or by some physiological shock permitting
nutrients to begin to enter, or the endogenous inhibitors to leach out of
the spore. A classic example is that dormancy of the ascospores of
Neurospora crassa is broken by a 30-minute heat shock at 60°C or
exposure to 0.12 mM furfural (C5H402). This
relates to natural physiology because N. crassa is one of the first
moulds to emerge on organic remains after bush fires. Clearly, the heat of
the fire will activate dormant spores; however, furfural is prepared by acid
distillation of the sugar xylose and, as many plant hemicelluloses contain
xylans, bush fires are likely also to produce furfural.

Prior to the emergence of a germ tube, fungal spores undergo
a process of swelling (spherical growth) during which spores
increase in diameter up to four times due mainly to uptake of water. During this
phase, the metabolic activity of the spore increases greatly and protein, DNA
and RNA production all increase rapidly. This is followed by the emergence of
one or more germ tubes (young hyphal tips) that extend away from the spore in
typical apically-polarised manner (Fig. 1).

Fig. 1. Time lapse drawings showing development of young germlings of
Fusarium venenatum and Aspergillus nidulans during the initial
12 hours of growth at 25°C on agar-solidified medium. Note that the majority
of the first-formed branches are oriented at close to 90° to the long axis
of the main germ tube hypha, as new hyphal tips grow directly away from
their parent hypha to explore the substratum.